Alpha B-crystallin (αBC) is a member of the small heat shock family of proteins that is found in high levels in the ocular lens. Along with alpha A, beta and γ-crystallin, αBC forms the major water soluble structural protein of the vertebrate ocular lens that produces the necessary refractive index. Alpha crystallins are also implicated as molecular chaperones where they are proposed to bind unfolded and denatured proteins thereby suppressing non-specific aggregation and maintaining lens transparency. Interestingly, mice null for αBC have normal lenses indicating that this crystallin is not essential for development of the transparent lens. In addition to the lens of the eyes, high levels of αBC is found in the adult heart and skeletal muscle, with lower expression in kidney, lung, CNS glia, liver and developing heart and somites.
αBC expression is associated with a number of pathological conditions. Increased levels of αBC is found in oncogenic malignancies and in CNS glia of various neurological diseases such as Alexander's disease, Creutzfeldt-Jacob disease, Alzheimer's disease, Parkinson's disease, Multiple Sclerosis, and neurotropic infections. Heat shock and transition metals can also induce the expression of this crystallin in primary astrocytes.
Multiple Sclerosis (MS) is an autoimmune disease of the CNS of unknown etiology that affects ˜400 000 Americans. In MS, myelin reactive T cells enter into the brain and spinal cord and mediate destruction of the myelin sheath surrounding neurons resulting in progressive motor dysfunction and eventual paralysis. Current treatment strategies include switching the pro-inflammatory Th1 T cell phenotype to an anti-inflammatory Th2 response, preventing encephalitogenic T cells from extravasating into the brain, inducing T cell tolerance, anergy or apoptosis, and repairing or replacing damaged CNS cells, such as neurons and oligodendrocytes.
The course of disease is highly varied and unpredictable. In most patients, especially when MS begins with optic neuritis, remissions can last months to >10 yr. However, some patients have frequent attacks and are rapidly incapacitated, although life span is shortened only in very severe cases.
Goals for therapy include shortening acute exacerbations, decreasing frequency of exacerbations, and relieving symptoms; maintaining the patient's ability to walk is particularly important. Acute exacerbations may be treated with brief courses of corticosteroids. However, although they may shorten acute attacks and perhaps slow progression, corticosteroids have not been shown to affect long-term outcome.
Immunomodulatory therapy decreases frequency of acute exacerbations and delays eventual disability. Immunomodulatory drugs include interferons (IFNs), such as IFN-β1b and IFN-β1a. Glatiramer acetate may also be used. Other potential therapies include the immunosuppressant methotrexate and Natalizumab, an anti-α4 integrin antibody that inhibits passage of leukocytes across the blood-brain barrier. Immunosuppressants such as mycophenolate and cyclophosphamide have been used for more severe, progressive MS but are controversial.
In addition to suppressing the pathological immune response it is important to protect CNS cells from further damage and to induce repair of injured cells since some cells such as neurons have few progenitors in the adult mammalian brain and are thus limiting.
Early studies in MS patients implied that αBC may have an autoantigen role in this disease. Myelin isolated from MS brains contained a single fraction that turned out to be αBC that was localized to oligodendrocytes and astrocytes and proved highly immunodominant to MS and control T cells by inducing proliferation and IFN-γ production. In large scale transcriptional profiling of MS brain lesions with a robot capable of sequencing genes Chabas et. al. (2001) Science 294, 1731-5 also found αBC to be the most abundant gene transcribed in early active MS. Three polymorphisms at positions C249G, C650G and A652G in the αBC gene have also been found to be associated with susceptibility to MS and disease expression (van Veen et al. (2003) Neurology 61, 1245-9). Further evidence for an autoantigen role of αBC in MS include increased proliferation of, and production of IL-2, IFN-γ and TNF from CD4 +T cell lines in response to αBC peptides from early active MS patients (Chou et al. (2004) J Neurosci Res 75, 516-23). It has been suggested that the protein is taken up for class II MHC-restricted presentation to T cells by local APC is these lesions.
The role of αBC in EAE and MS is discussed in, for example, van Stipdonk et al. (2000) J Neuroimmunol 103, 103-11; van Stipdonk et al. (2000) Cell Immunol 204, 128-34; Thoua et al. (2000) J Neuroimmunol 104, 47-57; and Sotgiu et al. (2003) Eur J Neurol 10, 583-6 (2003).
Recently a significant body of work has established an apoptotic role for αBC during stress. Alpha B crystallins were shown to protect cells from thermal, osmotic and oxidative insults, staurosporine, TNF, okadaic acid, hydrogen peroxide, calcimycin, and etoposide. In addition, αBC transgenic mice are protected against cardiomyocyte apoptosis and necrosis during myocardial ischemia and reperfusion.
This protection is apparently due to inhibition of caspase-3 activation. Normally, the mitochondrial and death receptor pathways would activate caspase 8 and 9 respectively that then converge to proteolytically activate the downstream executioner caspase 3. It appears that αBC inhibits the autoproteolytic maturation of the caspase 3 intermediate, p24, thereby inhibiting the apoptotic pathways. Other studies show that αBC interacts with Bax and Bcl-XS to prevent the translocation of these pro-apoptotic regulators into the mitochondria leading to abrogation of the downstream apoptotic events. In addition to its anti-apoptotic function recent work has also demonstrated an anti-inflammatory effect of α-crystallins. Pretreatment of mice with α-crystallin protected against silver nitrate neuroinflammation by decreasing GFAP, NF-κB expression in the neocortex, reversed intracellular calcium levels, acetylcholine esterase activity and depletion of glucose, and prevented nitric oxide, and lipid peroxide production in the brain.
A further elucidation of the role of αBC in inflammation is of great interest.